Beyond Being Beneficial—Symbiotic Microbes in Insect Pests

The long-standing arms race between plants and herbivores has led to the evolution of highly diverse mutual adaptation strategies, among which microbes play an indispensable role [1,2,3,4]. Certain plant-associated symbionts can help counteract insect herbivory and damage, while some insect-associated microbes can assist herbivorous insects in overcoming plant physical and chemical defenses [5,6]. Microbial involvement in plant–insect interactions can be direct, such as insect-associated microbes degrading plant defense compounds or plant symbionts directly killing insects [2,7,8]. Indirect involvement is also possible, as seen in the case of the gut bacteria in Dendroctonus valens, which regulate insect attack density by producing the aggregation pheromone verbenone, thereby affecting the host plant’s defense capability [9].

For humans, we have successfully domesticated over 2500 plant species into our food crops [10]. Insects that feed on these crops are classified as pests. Similarly, insects that damage trees are defined as forestry pests. Humans have long been searching for different ways to eliminate these pests. As mentioned earlier, numerous microorganisms inhabit the interior and exterior of insects, playing crucial roles in their hosts’ life cycles, often significantly enhancing their fitness [11,12]. Therefore, before utilizing them, we need to clarify the mechanisms by which these microorganisms are involved. Notably, these microbes, as a community, also engage in internal competition [13]. The balance among them is easily disrupted, and the abundance of different microorganisms can fluctuate with environmental changes [14,15]. When the host insect’s environment changes, such as with invasive insects, their associated microbiota also change. A recent study by Zhang et al. found that Hyphantria cunea can recruit gut microorganisms (Bacteroides, Blautia, and Coprococcus) to help adapt to a new host plant (Metasequoia glyptostroboides) in its newly invaded region (southern China) [16]. Other research has shown that silkworms feed on mulberry leaves to enrich the surface bacterium Mammaliicoccus sciuri, enhancing their ability to combat pathogenic fungi [17]. Whether through active or passive changes, some key microbes help insects improve hosts’ adaptability through their influence.

Thus, if we can identify the crucial microorganisms playing critical roles in the adaptability mechanisms of pests, we may find the “key” to preventing pests from damaging plants [18]. In other words, the role of symbiotic microbes in insect pests goes beyond merely being beneficial to the pests themselves from the perspective of humans or plant protection. This Special Issue in the journal Agronomy (ISSN 2073-4395) includes several studies attempting to identify such key microorganisms. For instance, a study focuses on screening for high-virulence Streptomyces with the ability to kill nematodes or plant pathogenic fungi, while the other one aims to find pivotal microbes affecting the growth of agricultural pests like Chilo suppressalis [19,20]. In summary, regardless of future technological advancements, exploring the role of microorganisms in plant–insect interactions and investigating their potential applications remains a crucial direction for research.